1. Field of the Invention
This invention relates to a method for the production of a porous cross-linked polymer which comprises a step of polymerizing a water-in-oil type high internal phase emulsion (hereinafter referred to briefly as "HIPE") and a step of after curing the HIPE in a very short period of time thereby producing a porous cross-linked polymer having continuous cells having communicating pores formed in the surface and the interior thereof (hereinafter referred to also as "open cells"). More particularly, this invention relates to a method for the production of a porous cross-linked polymer which can be widely applied to (1) liquid absorbent materials such as, for example, 1 core materials in disposable diapers to be used for absorbing body fluid such as urine and blood; and 2 absorbing materials of water, oil and organic solvent to be used for disposing of a waste oil a waste solvent and waste organic solvents; (2) energy absorbent materials such as, for example, sound insulating materials and heat insulators in automobiles and buildings to be used for absorbing sound and heat; and (3) chemical impregnating substrates such as, for example, products of domestic use impregnated with an aromatic agent, a detergent, a lustering agent, a surface protecting agent, and a flame-retarding agent.
2. Background Art
The term "HIPE" refers to an emulsion wherein the ratio of a water phase, i.e. a dispersed phase (inner phase), to an oil phase, i.e. an outer phase is not less than about 3/1. It is known to produce a porous cross-linked polymer material by polymerizing this HIPE. The porous cross-linked polymer material produced by using a foaming agent without preparing an HIPE (hereinafter occasionally referred to simply as "foam") is disposed to afford a foam of closed cells of a comparatively large diameter. In contrast, the method for producing a porous cross-linked polymer material from an HIPE (hereinafter occasionally referred to briefly as "HIPE method") excels in capability of producing a low-density foam of open cells having a minute diameter. Methods for producing porous cross-linked polymer materials from HIPE's are disclosed in the official gazette of U.S. Pat. No. 4,522,953, WO93/24,535 and U.S. Pat. No. 5,290,820, for example.
The official gazette of U.S. Pat. No. 4,522,953 discloses a method which comprises preparing an HIPE containing a water-soluble and/or an oil-soluble polymerization initiator and thermally polymerizing this HIPE at 50.degree. C. or 60.degree. C. for eight hours to 72 hours. The official gazette of WO93/24,535 discloses a method which produces a sheet-like porous cross-linked polymer material by placing an HIPE in a bag of film, reeling up the bag containing the HIPE, and causing the HIPE to cure as held in the bag and the official gazette of WO93/24,535 discloses a method which comprises forming a gel possessed of a prescribed dynamic modulus of elasticity in shear from an emulsion at a temperature of less than 65.degree. C. and thereafter polymerizing the gel at a temperature of not less than 70.degree. C. The invention of the official gazette of WO93/24,535 requires to form the gel because the emulsion is unstable at high temperatures and the formation of the gel enables the emulsion to acquire strength enough for the gel to be polymerized at the subsequent step. Further, for the purpose of obtaining a porous cross-linked polymer material in a short period of time, this invention contemplates incorporating in the emulsion a polymerization catalyst in an amount in the range of 0.005-15 wt % based on the weight of a polymerizable monomer and precuring the emulsion to effect the formation of a gel.
The methods disclosed in the official gazette of U.S. Pat. No. 4,522,953 and WO93/24,535, however, are deficient in efficiency of production because they entail unduly long periods of polymerization time. The invention of the official gazette of WO93/24,535 is claimed to attain the polymerization in a comparatively short period of time by a method comprising the steps of forming a gal possessed of a prescribed dynamic modulus of elasticity in shear from an emulsion at a temperature of less than 65.degree. C. and thereafter polymerizing the gel at a temperature of not less than 70.degree. C. The method, nevertheless, requires roughly several hours as the time of polymerization.
Generally, for the purpose of shortening the time of polymerization, such methods as increasing the amount of a polymerization initiator and promoting the decomposition of the initiator prove effective. The velocity of polymerization and the molecular weight of a polymer are, however, in inverse proportion. The reduction in the polymerization time entails a decline of the molecular weight of a polymer contained in the HIPE and this decline frequently results in degrading the qualities such as absorption properties and mechanical strength. In the production of a porous cross-linked polymer material, particularly an attempt to complete the polymerization in such a short span of time as within one hour brings a prominent degradation in performance. It is an object of this invention, therefore, to solve the technical problems of the prior art mentioned above and provide a method for the production of a porous cross-linked polymer material which effects polymerization of an HIPE in an exceptionally short period of time without impairing the stability of the HIPE.